A motor is identifiable as a complex function member where rotors, axes, bearings, stators, etc. are finished with various materials such as steels, non-ferrous metals, polymers in precision manners and assembled to convert electric energy into mechanical energy. In recent years, the mainstream of a motor is becoming a permanent-magnet type motor with a magnet carrying abilities to attract or repel other magnetic materials, and to permanently generate static magnetic fields without external energy. In a physical point of view, a point where a magnet is different form other magnetic materials is that the magnet keeps practicable magnetization after external magnetic fields are eliminated, and flux reversal (Demagnetization) will occur first when heat or relatively large reverse magnetic fields are applied causing decline of magnetization along therewith. Considering important property values of these magnets, energy density (BH)max can be named. This energy density (BH)max indicates the potential energy of a magnet per unit volume.
Here, ability of a magnet allowing strong attraction or repulsion does not necessarily lead to high-performability of the magnet depending on kinds of motors. However, according to Non-Patent Document 1, in relation with remanent flux density Br that is one of the basic properties of a magnet and motor constant KJ that is the barometer of a motor performance (KJ is a ratio between output torque KT and a square root √{square root over ( )}R of ohmic loss), when a motor diameter, a rotor diameter, a clearance, a soft magnetic material, a magnetic dimension, etc. are fixed, it is supposed to be able to obtain higher torque density along with increase of energy density (BH)max of a magnet in an inner rotor type brushless motor or a slotless (coreless) motor that applies an annular magnet to which the present invention is subjected.
However, considering the increase of the energy density (BH)max, in an outer rotor type brushless motor to which the present invention is subjected to, it would be difficult to obtain the higher torque density compared to the motors hereinabove described. In addition, since the stator core or the armature core of the above-described motors or a permanent-magnet-field-type DC motor is provided with a slot into which a winding is housed and teeth that partially form magnetic circuits, permeance is adapted to be altered along with rotation. Accordingly, the increase of energy density (BH)max of a magnet will raise torque pulsation, that is, cogging torque. The increase of the cogging torque may cause some disturbances such as hampering smooth rotation of a motor, magnifying vibration or noise of the motor or deteriorating rotation controllability.
Therefore, an outer rotor type motor in a slotless (coreless) structure where there is no permeance change along with rotation, and cogging torque is not basically generated is known. However, since the motor has an air-core winding in a clearance between a magnet and an iron core faced thereto, there is a problem as that magnetic resistance in a motor magnetic circuit is notably increased. Accordingly, in a motor magnet with the above structure, a magnet satisfying the following has been thus demanded. That is, along with the increase of so-called energy density (BH)max of a magnet, static magnetic fields that generate magnetic poles as a motor magnetic circuit are adapted to be not consumed as leakage flux the magnetic flux linkage of a stator core teeth or the air-core winding is enhanced, or the direction of anisotropy is continuously controlled in order to decrease the armature reaction of permanent-magnet-field-type DC motors.
Here, there can be found some studies working on the rare earth-iron based magnet that controls the direction of anisotropy as discussed hereinabove and the motor using the same.
For example, Non-Patent Document 2 is subjected to an inner rotor type brushless motor and applies a rare earth-iron based sintered magnet with high energy density where the thickness of the magnet is as thin as 1.2 mm, and its remanence Mr is 1 T. As shown in FIGS. 10(A) to 10(D), a single magnetic pole is composed of fragments where each of magnetic poles is divided into 2 to 5 pieces, and the direction of anisotropy (the direction of an easy magnetization axis) is stepwisely adjusted in every magnetic pole fragment. This is a so-called Halback Cylinder. Here, in FIGS., the subscripts (2) to (5) of a magnetic pole 51 indicate a number of the fragments where the magnetic pole 51 is divided into 2 to 5 pieces. Further, the direction of arrows for each fragment indicates the direction of anisotropy (the direction of the easy magnetization axis).
In consideration of an inner rotor type brushless motor with 12 poles and 18 slots using the above-structured magnetic pole, when determining a relation between a number N of the divided magnetic-pole fragments and cogging torque Tcog based on the values of cogging torques, it is possible to suppose that the power approximation of Tcog=61.753exp (−0.145 N) can be established. To be more specific, when an angle defied by a magnetized vector M in an optional mechanical radian φ and a magnetized vector relative to a tangent line of a magnetic pole in an outer circumferential direction is set to M θ, it suggests being ideal that the direction of anisotropy has continuous variations in a regular and precise manner between the magnetic poles. However, considering the rare earth-iron based sintered magnet with high energy density where its thickness is 1.2 mm, and remanence Mr is 1 T, it would be very difficult to obtain a rotor where the plurality of magnetic-pole fragments with anisotropy in different directions are prepared, the magnetic-pole fragments are regularly and finely arranged, and the plurality of magnetic poles are structured with high dimensional precision. Accordingly, it becomes extremely difficult to produce a rotor with a multipolar annular magnet in which to have integral times of magnetic poles, or the inner rotor type brushless motor using the same. Moreover, it would be easy to speculate that the above described rotor be in less economic performance.
On the other hand, Non-Patent Documents 3 and 4 noticed that the power approximation has been established between the divided magnetic-pole fragments N and the cogging torque Tcog that has been concerned in Non-Patent Document 2. Accordingly, Non-Patent Documents 3 and 4 disclose the manufacturing method of a rare earth-iron based magnet that continuously controls the direction of anisotropy in the same structure of the inner rotor type brushless motor, and disclose effects to reduce cogging torques of the motor based on a magnet that continuously controls the direction of anisotropy. Specifically, as the manufacturing method of a magnet, as shown in FIG. 11(A), an anomalously-outlined segment is prepared, the anomalously-outlined segment being as that an angle H θ defined by a homogeneously aligned magnetic field Hex being kept in a constant direction and tangent lines of the magnetic-pole fragments divided into 96 pieces in inner and outer peripheries is continuously varied from a vertical plane to an in-plane direction. Then, as shown in FIG. 11(B), by applying rheology based on plastic deformation, an arc-segmented magnet with energy density (BH)max of 155 to 158 kJ/m3 is achieved so as to provide a rotor for the inner rotor type brushless motor. Here, Hex and its arrow in FIG. indicate a uniformly aligned magnetic field and its direction; a line a-a′ indicates a tangent line in an outer periphery direction at an optional position; M and its arrow indicate a magnetized vector and its direction; H θ indicates an angle defined by a tangent line in a periphery direction and M; and M θ indicates an angle defined by a tangent line in a periphery direction and M. Here, M θ indicates the direction of anisotropy at an optional position, and H θ≈M θ can be established.
Based on Patent Documents 3 and 4 discussed hereinabove, defects of Non-Patent Document 2 in terms of manufacture of the magnetic poles seem to be solved. However, considering the outer rotor type motor or the permanent-magnet-field-type DC motor in the slotless (coreless) structure to which the present invention is subjected, the surface of the magnetic poles is an inner periphery but not an outer periphery. Accordingly, as shown in FIG. 12, compared to Non-Patent Document 3 that is subjected to the inner rotor type brushless motor, configuration to obtain anisotropic distributions will be remarkably differentiated. For example, in order to obtain an arc-segmented magnet (A-B-C-D) as shown in the coordinate of FIG. 12, Patent Documents 3 and 4 will perform plastic deformation from the position of coordinate value A-b(inner)-c(inner)-D by using rheology. However, in case of the outer rotor type brushless motor to which the present invention is subjected, a large deformation will be needed from the position of coordinate value A-b(outer)-c(outer)-D. Accordingly, compared to Non-Patent Documents 3 and 4, magnetic materials with prominently large deformability will be required. That is, reducing the volume fraction of the magnetic materials will be impossible while maintaining or improving energy density as disclosed in Non-Patent Documents 3 and 4, whereby the magnetic materials are mechanically impaired through deformation processes. Accordingly, the methods disclosed in Non-Patent Documents 3 and 4 cannot be directly applied.
As discussed hereinabove, compared to Non-Patent Documents 3 and 4, since the present invention is subjected to the outer rotor type brushless motor or the permanent-magnet-field-type DC motor that has pole surfaces on its inner periphery, not only can not be the method of Non-Patent Documents 3 and 4 directly applicable, yet larger energy density (BH)max is needed in order to expand magnetic resistance as a motor magnetic circuit, especially in a slotless structure.
Here, considering the study of rare earth-iron based magnetic materials regarding improvement of the above-described energy density (BH)max, it seems that R. W. Lee et al. are the first individuals to introduce that an isotropic Nd2Fe14B based bonded magnet with (BH)max of 72 KJ/m3 can be achieved by fixing a rapid solidification ribbon with (BH)max of 111 kJ/m3 with resin (referred to “R. W. Lee, E. G. Brewer, N. A. Schaffel, “Hot-pressed Neodymium-Iron-Boron magnets” IEEE Trans. Magn., Vol. 21, 1958 (1985)”). Since then, from the late of the 1980's to the present, the study of the isotropic rare earth magnetic materials, mainly concerned with rapid solidification of rare earth-iron based molten alloy, has been actively conducted. For example, including nanocomposite magnetic materials using an exchange coupling based on Nd2Fe14B-base or Sm2Fe17N3-base, or also based on a fine structure of the Nd2Fe14B-base or the Sm2Fe17N-base in addition to αFe, FeB or Fe3B-base, isotropic magnetic materials micro-controlling variable alloy structures are industrially applicable. Further, isotropic magnetic materials in different powder configurations are also industrially applicable. See, for example, Non-Patent Documents 5 to 8. Here, especially in Non-Patent Document 8, H. A. Davies et al. reported that (BH)max reaches to 220 kJ/m3 even in isotropy. However, (BH)max of isotropic magnetic materials industrially applicable is approximately 134 kJ/m3 m at the highest. Energy density (BH)max of isotropic Nd2Fe14B bonded magnets that have been typically applied to small motors with less than approximately several watts is approximately 80 kJ/m3 m or less. That is, since R. W. Lee et al. have produced the isotropic Nd2Fe14B based bonded magnet with (BH)max of 72 kJ/m3 with the ribbon with (BH)max x of 111 kJ/m3 in 1985, in view of advancement of (BH)max, the advancement has been less than even 10 kJ/m3.
Accordingly, it would be difficult to expect the torque densification of a motor to which the present invention is subjected while waiting the advancement of the isotropic magnetic materials that increase energy densities.
NON-PATENT DOCUMENT 1: “Application of high performance magnets in small motors” written by J. Schulze, Proc. of the 18th international workshop on high performance magnets and their applications, published in 2004, pp. 908-915
NON-PATENT DOCUMENT 2: “Comparison of brushless motors having halback magnetized magnets and shaped parallel magnetized magnets” written by Y. Pang, Z. Q. Zhu, S. Ruangsinchaiwanich and D. Howe, Proc. of the 18th international workshop on high performance magnets and their applications, published in 2004, pp. 400-407
NON-PATENT DOCUMENT 3: “Preparation method of rare earth bonded magnets with continuously controlled anisotropy directions” written by F. Yamashita, K. Kawamura, Y. Okada, H. Murakami, M. Ogushi, M. Nakano and H. Fukunaga, Journal of Applied Physics, 101, 09K522 (2007)
NON-PATENT DOCUMENT 4: “Composite bonded magnets with controlled anisotropy directions prepared by viscous deformation technique” written by F. Yamashita, K. Kawamura, Y. Okada, H. Murakami, M. Ogushi, M. Nakano and H. Fukunaga, J. Magnetism Magn. Mater., Vol. 316, Issue 2, published in 2007, pp. e101-e104
NON-PATENT DOCUMENT 5: “Recent developments in Nd—Fe—B powder” written by B. H. Rabin and B. M. Ma, 120th Topical Symposium of the Magnetic Society of Japan, published in 2001, pp. 23-28
NON-PATENT DOCUMENT 6: “Recent powder development at magnequench” written by B. M. Ma, Polymer Bonded Mangets 2002, published in 2002
NON-PATENT DOCUMENT 7: “Structure and magnetic properties of Nd2Fe14B/FexB-type nanocomposites prepared by strip casting” written by S. Hirosawa, H. Kanekiyo, T. Miyoshi, K. Murakami, Y. Shigemoto and T. Nishiuchi, 9th Joint MMM/INTERMAG, FG-05, published in 2004
NON-PATENT DOCUMENT 8: “Nanophase Pr and Nd/Pr based rare-earth-iron-boron alloys” written by H. A. Davies, J. I. Betancourt and C. L. Harland, Proc. of 16th Int. Workshop on Rare-Earth Magnets and Their Applications, published in 2000, pp. 485-495